It depends what you are trying to achieve. A proper repair on that is going to be time consuming, quite skilled and probably expensive. Not meaning to be rude about the boat, but if you paid a professional it probably wouldn't be worth it.

Flocoat is like "gel coat" (which is not the same as paint - but is the outside, tough, water proof coloured layer on the GRP). Gel coat won't "set" if it is exposed to air (it is normally put in the mould first, then the grp laid up on top), so for use when you are adding it as the final layer people use flocoat - which is a mix of gel and wax - the wax separates to the surface and keeps the air out.

Mat is held in place with resin. Its not a trivial project for your first ever trip into the world of fibreglassing.

I've bodged similar repairs in the past using the "car body filler" approach. You've got a lot of cracking around that too though - which need exploring and probably grinding back and repairing too. The main problems with simply filling a patch that size and location is:

(i) you won't get a good colour match to the surrounding gel
(ii) you probably won't get a very good shape match to the surrounds.
(iii) supporting material whilst filling a big area is tricky (especially as you probably have no access to the underside).
(iv) getting good adhesion to the neighbouring material can be a challenge - in some places this doesn't matter too much - but somewhere than can be easily bumped is likely to just pop the big lump of filler out in one lump.

Do you have access to the inside of the damage or is it in a sealed compartment?

If you have access life gets easier. If not its still repairable.

I'd probably only use the body filler option as a stop gap. Its probably not gonna last forever that way. Will the cause of the damage ever happen again?

As said - the damage needs ripping back with a grinder till u've got sound GRP. That may be a lot and sounds extreme but it needs to be done.

You then need to bevel the edges of the gel coat (the blue stuff). Any cracks that are only the gel can be cleaned out with a grinding bit on a dremel.

Some Chopped Strand Mat (CSM) will be needed. I'd say looking at the curve you'll need 300gram rather than 450. I'd suspect you treat the bow and stern as two seperate holes and get first shape of CSM on each and then an extra bit to bring the together. If you can get an arm inside you simply roughen the old GRP 2inches round the hole, cut CSM to fit that 2" area plus the hole then apply fibreglass resin (mixed with catalyst) to the roughened surface, then pop on the CSM and apply more resin. Allow to harden. Repeat for other asepect of the home then join the 2 together with a third 4" strip of CSM. Make sure you get rid of the air bubbles. Apply more layers of CSM to strengthen.

From the outside build up CSM to take the basic shape but keep below the overall surface. Finish with Gel/Flo Coat and get sanding.

If you can get a RAL colour from the manufacturer then someone like East Coast Fibre Glass will be able to get you the right colour Flow Coat in a tin of any size (500g / 1kg should be more than enough). BUT they will match the original colour not the UV faded colour so it will not be 100% perfect.

If you have no access to the inside you need to get something inside the hole to act as a former. Some people use a wire mesh that will hold roughly the required shape. Attach a thin string / wire to that so you can pull it against the hole. I've used cardboard but never for a hole that size always smaller. Basically you get the former into the space with string attached and then pull in agained the hole and shape it to do the forming you want. Release the strings a bit to give you some working room. Then apply CSM to the former and apply resin. Once ready you pull the strings again and keep the pressure on while the overlap inside the holes fixes to the sound shape. Cut strings at the CSM when set and then proceed as above applying further CSM to strengthen and build up the surface etc.

Fibre glass resin is temperature sensitive. Mixing ratio will vary according to your working temp. You want a decent pot life to get the resin on but not too long that you have to keep fingers crossed the strings don't break etc while waiting. Ideally some *GENTLE* heat applied will help that so you can work at 10C like today to get it on then pop a heater on nearby (don't go to extremes like a hair drier) and it will then speed things up.

You will need:
Angle Grinder
Sander - ideally an oscilating circular one

I would just add also protective gear...Angle grinder is very efficient to prepare the surface but it is notorious for creating and throwing the not so healthy dust all around close and far.
Would recommend a 3M(or similar) half face mask, using both particle and gas filter, it does not cost much but improves the air quality a lot both in preparation and when laminating.

Disposable/medical gloves are handy when playing with the resin I usually use a double or triple layer so when they get messy i just rip one layer away. I am sure it will be fine, good luck!

__________________

__________________
fun on a boat is inversely proportional to size...sort of anyway